Nitrous oxide in coastal waters

We determined atmospheric and dissolved nitrous oxide (N2O) in the surface waters of the central North Sea, the German Bight, and the Gironde estuary. The mean saturations were 104 ± 1% (central North Sea, September 1991), 101 ± 2% (German Bight, September 1991), 99 ± 1% (German Bight September 1992), and 132% (Gironde estuary, November 1991). To evaluate the contribution of coastal areas and estuaries to the oceanic emissions we assembled a compilation of literature data. We conclude that the mean saturations in coastal regions (with the exception of estuaries and regions with upwelling phenomena) are only slightly higher than in the open ocean. However, when estuarine and coastal upwelling regions are included, a computation of the global oceanic N2O flux indicates that a considerable portion (approximately 60%) of this flux is from coastal regions, mainly due to high emissions from estuaries. We estimate, using two different parameterizations of the air-sea exchange process, an annual global sea-to-air flux of 11–17 Tg N2O. Our results suggest a serious underestimation of the flux from coastal regions in widely used previous estimates

Nitrous oxide emissions from the Arabian Sea

Dissolved and atmospheric nitrous oxide (N2O) were measured on the legs 3 and 5 of the R/V Meteor cruise 32 in the Arabian Sea. A cruise track along 65°E was followed during both the intermonsoon (May 1995) and the southwest (SW) monsoon (July/August 1995) periods. During the second leg the coastal and open ocean upwelling regions off the Arabian Peninsula were also investigated. Mean N2O saturations for the oceanic regions of the Arabian Sea were in the range of 99–103% during the intermonsoon and 103–230% during the SW monsoon. Computed annual emissions of 0.8–1.5 Tg N2O for the Arabian Sea are considerably higher than previous estimates, indicating that the role of upwelling regions, such as the Arabian Sea, may be more important than previously assumed in global budgets of oceanic N2O emissions

Integrating biomass, sulphate and sea-salt aerosol responses into a microphysical chemical parcel model: implications for climate studies

Aerosols are known to influence significantly the radiative budget of the Earth. Although the direct effect (whereby aerosols scatter and absorb solar and thermal infrared radiation) has a large perturbing influence on the radiation budget, the indirect effect (whereby aerosols modify the microphysical and hence the radiative properties and amounts of clouds) poses a greater challenge to climate modellers. This is because aerosols undergo chemical and physical changes while in the atmosphere, notably within clouds, and are removed largely by precipitation. The way in which aerosols are processed by clouds depends on the type, abundance and the mixing state of the aerosols concerned. A parametrization with sulphate and sea-salt aerosol has been successfully integrated within the Hadley Centre general circulation model (GCM). The results of this combined parametrization indicate a significantly reduced role, compared with previous estimates, for sulphate aerosol in cloud droplet nucleation and, consequently, in indirect radiative forcing. However, in this bicomponent system, the cloud droplet number concentration, Nd (a crucial parameter that is used in GCMs for radiative transfer calculations), is a smoothly varying function of the sulphate aerosol loading. Apart from sea-salt and sulphate aerosol particles, biomass aerosol particles are also present widely in the troposphere. We find that biomass smoke can significantly perturb the activation and growth of both sulphate and sea-salt particles. For a fixed salt loading, Nd increases linearly with modest increases in sulphate and smoke masses, but significant nonlinearities are observed at higher non-sea-salt mass loadings. This non-intuitive Nd variation poses a fresh challenge to climate modellers

Small-scale mixing processes enhancing troposphere-to-stratosphere transport by pyro-cumulonimbus storms

Deep convection induced by large forest fires is an efficient mechanism for transport of aerosol particles and trace gases into the upper troposphere and lower stratosphere (UT/LS). For many pyro-cumulonimbus clouds (pyroCbs) as well as other cases of severe convection without fire forcing, radiometric observations of cloud tops in the thermal infrared (IR) reveal characteristic structures, featuring a region of relatively high brightness temperatures (warm center) surrounded by a U-shaped region of low brightness temperatures. We performed a numerical simulation of a specific case study of pyroCb using a non-hydrostatic cloud resolving model with a two-moment cloud microphysics parameterization and a prognostic turbulence scheme. The model is able to reproduce the thermal IR structure as observed from satellite radiometry. Our findings establish a close link between the observed temperature pattern and small-scale mixing processes atop and downwind of the overshooting dome of the pyroCb. Such small-scale mixing processes are strongly enhanced by the formation and breaking of a stationary gravity wave induced by the overshoot. They are found to increase the stratospheric penetration of the smoke by up to almost 30 K and thus are of major significance for irreversible transport of forest fire smoke into the lower stratosphere

Infrequent new particle formation over the remote boreal forest of Siberia

Aerosol particle number size distributions (PNSD) were investigated to verify, if extremely low-volatility organic vapors (ELVOC) from natural sources alone could induce new particle formation and growth events over the remote boreal forest region of Siberia, hundreds of kilometers away from significant anthropogenic sources. We re-evaluated observations determined at a height of 300 m of the remote observatory ZOTTO (Zotino Tall Tower Observatory, http://www.zottoproject.org). We found that new particle formation events occurred only on 11 days in a 3-year period, suggesting that homogeneous nucleation with a subsequent condensational growth could not be the major process, maintaining the particle number concentration in the planetary boundary layer of the remote boreal forest area of Siberia. © 2018 Elsevier Lt

Aerosol particle number size distributions and particulate light absorption at the ZOTTO tall tower (Siberia), 2006–2009

This paper analyses aerosol particle number size distributions, particulate absorption at 570 nm wavelength and carbon monoxide (CO) measured between September 2006 and January 2010 at heights of 50 and 300 m at the Zotino Tall Tower Facility (ZOTTO) in Siberia (60.8° N; 89.35° E). Average number, surface and volume concentrations are broadly comparable to former studies covering shorter observation periods. Fits of multiple lognormal distributions yielded three maxima in probability distribution of geometric mean diameters in the Aitken and accumulation size range and a possible secondary maximum in the nucleation size range below 25 nm. The seasonal cycle of particulate absorption shows maximum concentrations in high winter (December) and minimum concentrations in mid-summer (July). The 90th percentile, however, indicates a secondary maximum in July/August that is likely related to forest fires. The strongly combustion derived CO shows a single winter maximum and a late summer minimum, albeit with a considerably smaller seasonal swing than the particle data due to its longer atmospheric lifetime. Total volume and even more so total number show a more complex seasonal variation with maxima in winter, spring, and summer. A cluster analysis of back trajectories and vertical profiles of the pseudo-potential temperature yielded ten clusters with three levels of particle number concentration: Low concentrations in Arctic air masses (400–500 cm−3), mid-level concentrations for zonally advected air masses from westerly directions between 55° and 65° N (600–800 cm−3), and high concentrations for air masses advected from the belt of industrial and population centers in Siberia and Kazakhstan (1200 cm−3). The observational data is representative for large parts of the troposphere over Siberia and might be particularly useful for the validation of global aerosol transport models

Cloud condensation nuclei in polluted air and biomass burning smoke near the mega-city Guangzhou, China – Part 1: Size-resolved measurements and implications for the modeling of aerosol particle hygroscopicity and CCN activity

Atmospheric aerosol particles serving as Cloud Condensation Nuclei (CCN) are key elements of the hydrological cycle and climate. We measured and characterized CCN in polluted air and biomass burning smoke during the PRIDE-PRD2006 campaign from 1–30 July 2006 at a rural site ~60 km northwest of the mega-city Guangzhou in southeastern China. CCN efficiency spectra (activated fraction vs. dry particle diameter; 20–290 nm) were recorded at water vapor supersaturations (S) in the range of 0.068% to 1.27%. The corresponding effective hygroscopicity parameters describing the influence of particle composition on CCN activity were in the range of κ≈0.1–0.5. The campaign average value of κ=0.3 equals the average value of κ for other continental locations. During a strong local biomass burning event, the average value of κ dropped to 0.2, which can be considered as characteristic for freshly emitted smoke from the burning of agricultural waste. At low S (≤0.27%), the maximum activated fraction remained generally well below one, indicating substantial portions of externally mixed CCN-inactive particles with much lower hygroscopicity – most likely soot particles (up to ~60% at ~250 nm). The mean CCN number concentrations (NCCN,S) ranged from 1000 cm−3 at S=0.068% to 16 000 cm−3 at S=1.27%, which is about two orders of magnitude higher than in pristine air. Nevertheless, the ratios between CCN concentration and total aerosol particle concentration (integral CCN efficiencies) were similar to the ratios observed in pristine continental air (~6% to ~85% at S=0.068% to 1.27%). Based on the measurement data, we have tested different model approaches for the approximation/prediction of NCCN,S. Depending on S and on the model approach, the relative deviations between observed and predicted NCCN,S ranged from a few percent to several hundred percent. The largest deviations occurred at low S with a simple power law. With a Köhler model using variable κ values obtained from individual CCN efficiency spectra, the relative deviations were on average less than ~10% and hardly exceeded 20%, confirming the applicability of the κ-Köhler model approach for efficient description of the CCN activity of atmospheric aerosols. Note, however, that different types of κ-parameters must be distinguished for external mixtures of CCN-active and -inactive aerosol particles (κa, κt, κcut). Using a constant average hygroscopicity parameter (κ=0.3) and variable size distributions as measured, the deviations between observed and predicted CCN concentrations were on average less than 20%. In contrast, model calculations using variable hygroscopicity parameters as measured and constant size distributions led to much higher deviations: ~70% for the campaign average size distribution, ~80% for a generic rural size distribution, and ~140% for a generic urban size distribution. These findings confirm earlier studies suggesting that aerosol particle number and size are the major predictors for the variability of the CCN concentration in continental boundary layer air, followed by particle composition and hygroscopicity as relatively minor modulators. Depending on the required and applicable level of detail, the information and parameterizations presented in this study should enable efficient description of the CCN activity of atmospheric aerosols in detailed process models as well as in large-scale atmospheric and climate models

Hygroscopicity distribution concept for measurement data analysis and modeling of aerosol particle mixing state with regard to hygroscopic growth and CCN activation

This paper presents a general concept and mathematical framework of particle hygroscopicity distribution for the analysis and modeling of aerosol hygroscopic growth and cloud condensation nucleus (CCN) activity. The cumulative distribution function of particle hygroscopicity, H(κ, Dd) is defined as the number fraction of particles with a given dry diameter, Dd, and with an effective hygroscopicity parameter smaller than the parameter κ. From hygroscopicity tandem differential mobility analyzer (HTDMA) and size-resolved CCN measurement data, H(κ, Dd) can be derived by solving the κ-Köhler model equation. Alternatively, H(κ, Dd) can be predicted from measurement or model data resolving the chemical composition of single particles. A range of model scenarios are used to explain and illustrate the concept, and exemplary practical applications are shown with HTDMA and CCN measurement data from polluted megacity and pristine rainforest air. Lognormal distribution functions are found to be suitable for approximately describing the hygroscopicity distributions of the investigated atmospheric aerosol samples. For detailed characterization of aerosol hygroscopicity distributions, including externally mixed particles of low hygroscopicity such as freshly emitted soot, we suggest that size-resolved CCN measurements with a wide range and high resolution of water vapor supersaturation and dry particle diameter should be combined with comprehensive HTDMA measurements and size-resolved or single-particle measurements of aerosol chemical composition, including refractory components. In field and laboratory experiments, hygroscopicity distribution data from HTDMA and CCN measurements can complement mixing state information from optical, chemical and volatility-based techniques. Moreover, we propose and intend to use hygros

Water uptake by biomass burning aerosol at sub- and supersaturated conditions: closure studies and implications for the role of organics

We investigate the CCN activity of freshly emitted biomass burning particles and their hygroscopic growth at a relative humidity (RH) of 85%. The particles were produced in the Mainz combustion laboratory by controlled burning of various wood types. The water uptake at sub- and supersaturations is parameterized by the hygroscopicity parameter, κ (c.f. Petters and Kreidenweis, 2007). For the wood burns, κ is low, generally around 0.06. The main emphasis of this study is a comparison of κ derived from measurements at sub- and supersaturated conditions (κG and κCCN), in order to see whether the water uptake at 85% RH can predict the CCN properties of the biomass burning particles. Differences in κGand κCCN can arise through solution non-idealities, the presence of slightly soluble or surface active compounds, or non-spherical particle shape. We find that κG and κCCN agree within experimental uncertainties (of around 30%) for particle sizes of 100 and 150 nm; only for 50 nm particles is κCCN larger than κG by a factor of 2. The magnitude of this difference and its dependence on particle size is consistent with the presence of surface active organic compounds. These compounds mainly facilitate the CCN activation of small particles, which form the most concentrated solution droplets at the point of activation. The 50 nm particles, however, are only activated at supersaturations higher than 1% and are therefore of minor importance as CCN in ambient clouds. By comparison with the actual chemical composition of the biomass burning particles, we estimate that the hygroscopicity of the water-soluble organic carbon (WSOC) fraction can be represented by a κWSOC value of approximately 0.2. The effective hygroscopicity of a typical wood burning particle can therefore be represented by a linear mixture of an inorganic component with κ ≅ 0.6, a WSOC component with κ ≅ 0.2, and an insoluble component with κ = 0

Biomass burning emissions estimated with a global fire assimilation system based on observed fire radiative power

The Global Fire Assimilation System (GFASv1.0) calculates biomass burning emissions by assimilating Fire Radiative Power (FRP) observations from the MODIS instruments onboard the Terra and Aqua satellites. It corrects for gaps in the observations, which are mostly due to cloud cover, and filters spurious FRP observations of volcanoes, gas flares and other industrial activity. The combustion rate is subsequently calculated with land cover-specific conversion factors. Emission factors for 40 gas-phase and aerosol trace species have been compiled from a literature survey. The corresponding daily emissions have been calculated on a global 0.5° × 0.5° grid from 2003 to the present. General consistency with the Global Fire Emission Database version 3.1 (GFED3.1) within its accuracy is achieved while maintaining the advantages of an FRP-based approach: GFASv1.0 makes use of the quantitative information on the combustion rate that is contained in the FRP observations, and it detects fires in real time at high spatial and temporal resolution. GFASv1.0 indicates omission errors in GFED3.1 due to undetected small fires. It also exhibits slightly longer fire seasons in South America and North Africa and a slightly shorter fire season in Southeast Asia. GFASv1.0 has already been used for atmospheric reactive gas simulations in an independent study, which found good agreement with atmospheric observations. We have performed simulations of the atmospheric aerosol distribution with and without the assimilation of MODIS aerosol optical depth (AOD). They indicate that the emissions of particulate matter need to be boosted by a factor of 2–4 to reproduce the global distribution of organic matter and black carbon. This discrepancy is also evident in the comparison of previously published top-down and bottom-up estimates. For the time being, a global enhancement of the particulate matter emissions by 3.4 is recommended. Validation with independent AOD and PM10 observations recorded during the Russian fires in summer 2010 show that the global Monitoring Atmospheric Composition and Change (MACC) aerosol model with GFASv1.0 aerosol emissions captures the smoke plume evolution well when organic matter and black carbon are enhanced by the recommended factor. In conjunction with the assimilation of MODIS AOD, the use of GFASv1.0 with enhanced emission factors quantitatively improves the forecast of the aerosol load near the surface sufficiently to allow air quality warnings with a lead time of up to four days